1. Field of the Invention
The present invention relates to an optical disc apparatus, and more particularly to an optical disc apparatus with a tilt angle adjustment function for an optical pickup.
2. Description of the Related Art
In an optical disc, an optical pickup for irradiating a laser beam onto an optical disc rotating on a turntable so as to read a signal or signals recorded on the optical disc is always required to irradiate the laser beam perpendicularly to a surface of the optical disc for the purpose of maintaining high reading accuracy. An angle of an optical axis of the laser beam relative to the surface of the optical disc is referred to as a tilt angle. An optical disc apparatus with an exclusive tilt sensor detects the tilt angle by the tilt sensor so as to obtain a proper tilt angle.
However, the use of an exclusive tilt sensor causes drawbacks, for example, that a base for mounting an optical pickup is required to have space for mounting the tilt sensor, and that an exclusive circuit for the tilt sensor causes a cost increase. For this reason, an optical disc apparatus has been developed that does not use an exclusive tilt sensor, but can detect an optimum tilt angle based on a reproduction signal of a reflected laser beam which the optical pickup receives. For example, Japanese Laid-open Patent Publication 2002-342963 discloses an optical disc apparatus which measures amplitude values of a reproduction signal of a reflected laser beam at multiple varied angles at predetermined angular intervals of an optical pickup relative to an optical disc (tilt angles), and which approximates the relationship between the amplitude value and the tilt angle by a quadratic function using the least squares method, so as to find an optimum tilt angle.
Japanese Laid-open Patent Publication 2004-95035 discloses an optical disc apparatus to obtain a proper tilt angle even for an unrecorded optical disc, by using reflected laser beams from land prepits on the optical disc, and by detecting a tilt angle to give a maximum signal amplitude of reflected laser beam and considering an offset amount, so as to find an optimum tilt angle. Furthermore, Japanese Laid-open Patent Publication 2000-311369 discloses an optical disc apparatus which finds an optimum tilt angle, using a tracking error signal detected by an optical pickup. In both of the latter two optical disc apparatus, quadratic function approximation is used to find or calculate a maximum value in obtained signal amplitudes.
As described above, conventional optical disc apparatus for finding an optimum tilt angle based on a reproduction signal of a reflected laser beam use quadratic function approximation in order to find a maximum amplitude value of e.g. a reproduction signal. However, experiments performed by the present inventor have indicated that the optimum tilt angle (maximum point) found by the quadratic function approximation slightly (more or less) shifts or deviates from an optimum tilt angle found by actually reproducing an optical disc. The experiments have further indicated that if a so-called “surface-vibrating” optical disc which much vibrates in the thickness direction during rotation is used, the shift is likely to be large.